Conventionally, for a boiler used in a high temperature environment, a chemical plant, and the like, a so-called “18-8 type austenitic stainless steel” such as SUS 304H, SUS 316H, SUS 321H, SUS 347H, and the like has been used as an equipment material.
However, in recent years, the conditions under which this equipment was used in a high temperature environment have become extremely severe, and therefore the performance requirements of material to be used have become stringent; under these circumstances, the above-described 18-8 type austenitic stainless steel, having been used conventionally, has become remarkably insufficient in high temperature strength, especially creep rupture strength. Therefore, in order to solve the said problem, an austenitic stainless steel, with improved creep rupture strength, has been developed by containing proper amounts of various elements.
On the other hand, nowadays, in the field of a boiler for thermal power generation, for example, a project is underway to raise steam temperature, which has conventionally been about 600° C. at the most, to 700° C. or higher. In this case, the temperature of a member to be used far exceeds 700° C., and therefore, even if the above-described newly developed austenitic stainless steel is used, the creep rupture strength and corrosion resistance are insufficient.
Generally, in order to improve the corrosion resistance, it is effective to increase the content of Cr in the steel. However, in the case where the Cr content is increased, for example, as seen in SUS 310S which contains about 25 mass % of Cr, the creep rupture strength at 600 to 800° C. rather becomes lower than that of 18-8 type stainless steels, and the toughness is deteriorated due to the precipitation of σ phase. Further, even if the Cr content is increased, about 25 mass % of Cr cannot provide sufficient corrosion resistance in a severe corrosive environment.
Thus, the Patent Documents 1 to 7 disclose heat resistant alloys in which the contents of Cr and Ni are increased, and moreover one or more kinds of Mo and W are contained in order to improve the creep rupture strength as high temperature strength.
Further, in order to meet the increasingly stringent requirements for high temperature strength characteristics, especially the requirements for creep rupture strength, the Patent Document 8 discloses a heat resistant alloy which contains, by mass %, 28 to 38% of Cr and 30 to 50% of Ni, and the Patent Documents 9 to 14 disclose heat resistant alloys which contain, by mass %, 28 to 38% of Cr and 35 to 60% of Ni. For all of the heat resistant alloys proposed in the Patent Documents 8 to 14, the creep rupture strength is further improved by utilizing the precipitation of α-Cr phase of a body-centered cubic structure consisting mainly of Cr.